Vacuum booster apparatus and a brake apparatus

ABSTRACT

A vacuum booster apparatus used for a brake apparatus positively decreases a degree of opening of a throttle valve when it is required while an unnecessary decrease in the degree of opening of the throttle valve is positively prevented. A first pressure chamber of a vacuum booster is connected to an intake pipe of the engine on a downstream side of a throttle valve. A second pressure chamber of the vacuum booster is selectively connectable to one of the first pressure chamber and atmosphere. The vacuum booster boosts an operating force applied to a brake operating member based on a pressure difference between the first pressure chamber and the second pressure chamber. A throttle-opening-degree control device controls a negative pressure in the first pressure chamber by controlling the degree of opening of the throttle valve. The throttle-opening-degree control device decreases the degree of opening of the throttle valve only for a limited time when the negative pressure in the first pressure chamber is smaller than a predetermined necessary negative pressure determined by an operating-force relating amount related to the operating force applied to the brake operating member.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a vacuum booster apparatus and abrake apparatus including a vacuum booster.

[0003] 2. Description of the Related Art

[0004] Japanese Laid-Open Patent Application No.8-164840 discloses avacuum booster apparatus provided with a vacuum booster. The vacuumbooster comprises a first pressure chamber which is connected to anintake line of an engine at a position downstream of a throttle valveand a second pressure chamber which is selectively connectable to eitherthe first pressure chamber or atmosphere. The vacuum booster boosts anoperational force applied to a brake-operating member based on adifference between a pressure in the first pressure chamber and apressure in the second pressure chamber. The vacuum booster apparatusalso includes a throttle-opening-degree control apparatus whichincreases a negative pressure in the first pressure chamber bydecreasing a degree of opening of the throttle valve. In theabove-mentioned vacuum booster apparatus, when the negative pressure inthe first pressure chamber becomes less than a predetermined negativepressure, the degree of opening of the throttle valve is decreased so asto maintain the negative pressure in the first pressure chamber at apressure higher than the predetermined negative pressure. Accordingly,since the negative pressure in the first pressure chamber is maintainedhigher than the predetermined negative pressure whether or not thebrake-operating member is being operated, a boost limit is preventedfrom being decreased. The boost limit is represented by a mastercylinder pressure at a time when a boost action reaches a limit.

[0005] The above-mentioned engine having the throttle valve is adirect-injection-type gasoline engine in which gasoline is directlyinjected into cylinders. In this type of engine, a uniform combustion isperformed when a load applied to the engine is large, that is, when anengine speed is high. On the other hand, when a load applied to theengine is not very large, that is, when the engine speed is medium orlow, a stratified charge combustion is performed. In the uniformcombustion, a combustion is achieved at a normal air-fuel ratio, thatis, a stoichiometric air-fuel ratio. In the stratified chargecombustion, an ultra lean burn occurs at a high air-fuel ratio of about25 to 50. According to the ultra lean burn, a rate of fuel consumptionis reduced which results in energy saving. When the engine load ismedium or low, a necessary drive torque can be output even when theultra lean burn occurs. Since, normally, the engine load is not verylarge when the brake operating member is being operated, the ultra leanburn is performed. In such the case, a degree of opening of the throttlevalve is maintained at a relatively large degree so as to intake a largeamount of air.

[0006] If the degree of opening of the throttle valve is maintained at arelatively large degree, the negative pressure in the first pressurechamber tends to be decreased. Accordingly, in the conventional vacuumbooster apparatus, the degree of opening of the throttle valve isdecreased when the negative pressure in the first pressure chamber isless than the predetermined negative pressure. When the degree ofopening of the throttle valve is decreased, a state of combustion shouldbe changed from the ultra lean burn to the uniform combustion. This isbecause an unstable combustion causing misfire occurs when the degree ofopening of the throttle valve is decreased. Additionally, the changefrom a state of the ultra lean burn to a state of the uniform combustionis performed stepwisely (gradually). If an output torque is the same, anamount of injection of fuel, the degree of opening of the throttle valveand a degree of opening of a swirl port are considerably differentbetween the state of the ultra lean burn and the state of the uniformcombustion. Accordingly, when the state of the ultra lean burn ischanged to the uniform combustion, these conditions are considerablychanged, which results in deterioration of drivability. Similarly, thereturn from the state of the uniform combustion to the state of theultra lean burn is performed stepwisely. As mentioned above, when thedegree of opening of the throttle valve is decreased when the ultra leanburn is being performed, it is required to stepwisely or graduallychange the ultra lean burn to the uniform combustion. This causesdeterioration in a rate of fuel consumption. Accordingly, it is notpreferable to decrease the degree of opening of the throttle valve so asto maintain a negative pressure in the first pressure chamber in a statein which the ultra lean burn is performed. Additionally, a frequency ofsuch a change in the state of combustion should be as small as possible.On the other hand, when a braking operation is not being performed,there is a possibility that the uniform combustion is performed. In sucha case, the rate of fuel consumption is not decreased by much even ifthe degree of opening of the throttle valve is decreased.

[0007] If the predetermined negative pressure at which the degree ofopening of the throttle valve is decreased can be decreased, that is, ifthe predetermined negative pressure can be set closer to an atmosphericpressure, a frequency of the degree of opening of the throttle valvebeing decreased can be decreased. However, in such a case, the limit ofboost is decreased. On the other hand, if the predetermined negativepressure is increased, that is, if the predetermined negative pressureis set closer to an absolute vacuum, the limit of boost can beincreased. That is, the boosting function can be provided until anoperational force applied to the brake-operating member is large.However, in such a case, the frequency of a degree of opening of thethrottle valve being decreased is increased. Additionally, since thevacuum booster cannot reach the limit of boost even if the negativepressure is less than the predetermined negative pressure in a rangewhere an operational force applied to the brake-operating member issmall, the degree of opening of the throttle valve is decreased despitethat the negative pressure is not required to be increased. As discussedabove, in the above-mentioned vacuum booster apparatus, it is difficultto decrease a frequency of the degree of opening of the throttle valvebeing decreased while a necessary negative pressure is maintained in thefirst pressure chamber.

[0008] In the above-mentioned patent document, a vacuum boosterapparatus which can decrease the frequency of the degree of opening ofthe throttle valve being decreased during a braking operation is alsodisclosed. This vacuum booster apparatus includes the above-mentionedvacuum booster and a throttle-valve-opening control apparatus. Thethrottle-valve-opening control apparatus increases a negative pressurein the first pressure chamber by decreasing the degree of opening of thethrottle valve when the brake-operating member is continuously operatedfor a period longer than a predetermined period and when the negativepressure in the first pressure chamber is less than the predeterminednegative pressure. In this vacuum booster apparatus, the degree ofopening of the throttle valve is always decreased when thebrake-operating member is continuously operated for a period longer thanthe predetermined period and when the negative pressure in the firstpressure chamber is less than the predetermined negative pressure.However, when an operational force applied to the brake-operating memberis small, the vacuum booster does not reach the limit of boost even ifthe braking operation continues for a long time and the negativepressure is small. In such a case, the negative pressure is not requiredto be increased. Additionally, when the operational force is large, thevacuum booster may reach the limit of boost even if the brakingoperation continues for only a short time. In such a case, the negativepressure should be increased. As discussed above, in the above-mentionedvacuum booster, the degree of opening of the throttle valve may beunnecessarily decreased or not decreased when it is required.

SUMMARY OF THE INVENTION

[0009] It is a general object of the present invention to provide avacuum booster apparatus and a brake apparatus in which theabove-mentioned problems are eliminated.

[0010] A more specific object of the present invention is to provide avacuum booster apparatus and a brake apparatus which positivelydecreases a degree of opening of a throttle valve when it is requiredwhile an unnecessary decrease in the degree of opening of the throttlevalve is positively prevented.

[0011] In order to achieve the above-mentioned objects, there isprovided according to one aspect of the present invention a vacuumbooster apparatus adapted to be used for a brake apparatus provided in avehicle having an internal combustion engine, the vacuum boosterapparatus comprising:

[0012] a vacuum booster having a first pressure chamber and a secondpressure chamber, the first pressure chamber being connected to anintake pipe of the engine on a downstream side of a throttle valve, thesecond pressure chamber being selectively connectable to one of thefirst pressure chamber and atmosphere, the vacuum booster boosting anoperating force applied to a brake operating member based on a pressuredifference between the first pressure chamber and the second pressurechamber; and

[0013] a throttle-opening-degree control device controlling a negativepressure in the first pressure chamber by controlling a degree ofopening of the throttle valve,

[0014] wherein the throttle-opening-degree control device includeslimited-time opening-degree decreasing means for decreasing the degreeof opening of the throttle valve only for a limited time when thenegative pressure in the first pressure chamber is smaller than apredetermined necessary negative pressure determined by an operatingforce relating amount related to the operating-force applied to thebrake operating member.

[0015] According to the present invention, the operating force relatingamount is related to the operating force applied to the brake operatingmember such as a brake pedal. That is, the operating force relatingamount includes the operating force itself and an amount in which theoperating force is reflected such as a travel of the brake operatingmember or a master cylinder pressure.

[0016] In the vacuum booster apparatus according to the presentinvention, the predetermined necessary negative pressure is determinedin accordance with the operating force relating amount that relates tothe operating force applied to the brake operating member. That is, thepredetermined necessary negative pressure is stepwisely or graduallyincreased as the operating-force relating amount is increased.Additionally, the degree of opening of the throttle valve is decreasedonly when the negative pressure in the first pressure chamber is smallerthan the predetermined necessary negative pressure.

[0017] The vacuum booster is closer to its boost limit when theoperating-force relating amount is large than when the operating-forcerelating amount is small. Accordingly, when the operating-force relatingamount is large, the negative pressure in the first pressure chamber ispreferably increased so as to increase a boost limit of the vacuumbooster so that the vacuum booster is prevented from reaching its boostlimit. On the other hand, when the operating force relating amount issmall, there is no need to increase the boost limit. That is, it is notrequired to increase the negative pressure in the first pressurechamber. According to the vacuum booster apparatus of the presentinvention, since the predetermined necessary negative pressure isdetermined in accordance with the operating-force relating amount, thedegree of opening of the throttle valve can be decreased only when it isnecessary.

[0018] In the vacuum booster apparatus according to the presentinvention, the limited-time opening-degree decreasing means may includenecessary-negative-pressure determining means for determining thepredetermined necessary negative pressure in accordance with anoperating-force relating amount related to the operating force appliedto the brake operating member.

[0019] The predetermined necessary negative pressure may be determinedby the operating-force relating amount alone or with other conditions ofa braking operation such as a period of operating the brake operatingmember and a rate of increase in the operating-force relating amount.

[0020] Additionally, in the vacuum booster apparatus according to thepresent invention, the limited-time opening degree decreasing means mayinclude means for determining the predetermined necessary negativepressure in accordance with a rate of increase in an operating-forcerelating amount related to the operating force applied to the brakeoperating member.

[0021] According to this invention, the predetermined necessary negativepressure is determined according to the operating force relating amountand the rate of increase in the operating-force relating amount. Thatis, the predetermined necessary negative pressure is increased as therate of increase is increased. Thus, if the operating-force relatingamount is the same, the predetermined necessary negative pressure whenthe rate of increase in the operating force relating amount is large isset to be greater than when the rate of increase in the operating-forcerelating amount is small.

[0022] When the rate of increase in the operating-force relating amountis large, a rate of evacuation of air in the first pressure chamber ispreferably increased so as to maintain an appropriate operational speedof the vacuum booster. On the other hand, when the rate of increase inthe operating-force relating amount is small, there is no need toincrease the predetermined necessary negative pressure. Accordingly,when the operating-force relating amount is the same, the predeterminednecessary negative pressure when the rate of increase in theoperating-force relating amount is large is preferably set to be greaterthan when the rate of increase in the operating force relating amount issmall.

[0023] Additionally, the rate of increase in the operating forcerelating amount is set to be a large value when it is required torapidly decrease a speed of a vehicle. In such a case, a possibility forthe vacuum booster reaching it boost limit is high. Thus, it ispreferable to increase the negative pressure in the first pressurechamber. On the other hand, when the rate of increase in theoperating-force relating amount is small, the possibility for the vacuumbooster reaching its boost limit is low. In this case, there is noproblem if the negative pressure in the first pressure chamber is small.Particularly, when an operating force applied to the brake operatingmember is released, the rate of increase in the operating-force relatingamount becomes a negative value. In this case, the possibility for thevacuum booster reaching its boost limit is low. That is, a margin to theboost limit when the rate of increase in the operating-force relatingamount is large is smaller than when the rate of increase in theoperating-force relating amount is small.

[0024] Additionally, in the vacuum booster apparatus according to thepresent invention, the limited-time opening degree decreasing means maydecrease the degree of opening of the throttle valve only when a boostlimit of the vacuum booster is smaller than an operational state valuewhich is determined according to an operating-force relating amountrelated to the operating force applied to the brake operating member.

[0025] The boost limit of the vacuum booster is equal to the mastercylinder pressure at a time when a boost action of the vacuum boosterreaches its limit. Accordingly, the operational state value can be themaster cylinder pressure as the operating force relating amount at thattime. Additionally, the operational state value can be set as othervalues such as the operating-force applied to the brake operating membercorresponding to the master cylinder pressure or a travel of the brakeoperating member. Further, the operational state value can be a valuedetermined by the operating-force relating amount and the rate ofincrease in the operating force relating amount.

[0026] The boost limit of the vacuum booster is determined by a pressuredifference between the first pressure chamber and the second pressurechamber when the pressure in the second pressure chamber reaches anatmospheric pressure. The boost limit is increased as the pressuredifference is increased. The pressure difference is increased as thenegative pressure in the first pressure chamber is increased.Accordingly, the boost limit is increased as the negative pressure inthe first pressure chamber is increased. A level of the negativepressure in the first pressure chamber needed for increasing the boostlimit to be greater than the operational state value is determined by alevel of the operational state value, and such a negative pressure isreferred to as a boosting-time necessary minimum negative pressure. Ifthe degree of opening of the throttle valve is decreased only when theboost limit is smaller than the operational state value, the negativepressure in the first pressure chamber can be maintained to be greaterthan the boosting-time necessary minimum negative pressure. Thus,measures should be taken for a decrease in the degree of opening of thethrottle valve only when the negative pressure in the first pressurechamber is smaller than the boosting-time necessary minimum negativepressure.

[0027] Additionally, there is provided according to another aspect ofthe present invention a vacuum booster apparatus adapted to be used fora brake apparatus provided in a vehicle having an internal combustionengine, the vacuum booster apparatus comprising:

[0028] a vacuum booster having a first pressure chamber and a secondpressure chamber, the first pressure chamber being connected to anintake pipe of the engine on a downstream side of a throttle valve, thesecond pressure chamber being selectively connectable to one of thefirst pressure chamber and atmosphere, the vacuum booster boosting anoperating force applied to a brake operating member based on a pressuredifference between the first pressure chamber and the second pressurechamber; and

[0029] a throttle-opening-degree control device controlling a degree ofopening of the throttle valve in accordance with a negative pressure inthe first pressure chamber and at least one of an operating-forcerelating amount related to the operating force applied to the brakeoperating member and a rate of increase in the operating-force relatingamount.

[0030] In the vacuum booster apparatus according to this invention, thedegree of the throttle valve is controlled based on the negativepressure in the first chamber and at least one of the operating-forcerelating amount and the rate of increase in the operating-force relatingamount. For example, the degree of opening of the throttle valve may bedecreased when the negative pressure in the first pressure chamber issmaller than a preset negative pressure and when the operating-forcerelating amount is greater than a preset amount, or when theoperating-force relating amount is greater than a preset amount and therate of increase in the operating-force relating amount is greater thana preset rate. In any case, the degree of opening of the throttle valveis decreased only when the negative pressure in the first pressurechamber is required to be increased. Thus, a frequency of operations fordecreasing the degree of opening of the throttle valve can be decreased.

[0031] Additionally, there is provided according another aspect of thepresent invention a vacuum booster apparatus adapted to be used for abrake apparatus provided in a vehicle having an internal combustionengine, the vacuum booster apparatus comprising:

[0032] a vacuum booster having a first pressure chamber and a secondpressure chamber, the first pressure chamber being connected to anintake pipe of the engine on a downstream side of a throttle valve, thesecond pressure chamber being selectively connectable to one of thefirst pressure chamber and atmosphere, the vacuum booster boosting anoperating force applied to a brake operating member based on a pressuredifference between the first pressure chamber and the second pressurechamber; and

[0033] a throttle-opening-degree control device controlling a negativepressure in the first pressure chamber by controlling a degree ofopening of the throttle valve, wherein the throttle-opening-degreecontrol device includes large-change-time opening degree decreasingmeans for decreasing the degree of opening of the throttle valve as arate of increase in an operating-force relating amount related to theoperating force applied to the brake operating member is increased.

[0034] The negative pressure in the first pressure chamber needed to beincreased more when the rate of increase in the operating-force relatingamount is large as compared to when the rate of increase is small.

[0035] In the conventional vacuum booster apparatus, the degree ofopening of the throttle valve is decreased when an operation of thebrake operating member continues longer than a preset time and when thenegative pressure in the first pressure chamber is smaller than a presetnegative pressure. However, when a rapid braking operation is performed,that is, when the rate of increase in the operating force relatingamount is large, the negative pressure in the first pressure chambermust be increased even if the operation of the brake operating membercontinues a short time. Yet, in such a case, the degree of opening ofthe throttle valve is not decreased. n the other hand, when the rate ofincrease in the operating-force relating amount is small, that is, forexample, when the operating force applied to the brake operating memberis released (the rate of increase is a negative value), the degree ofopening of the throttle valve is decreased although the negativepressure in the first pressure chamber could be small. In order toeliminate such an inconvenience, in the vacuum booster apparatusaccording to this invention, the degree of opening of the throttle valvewhen the rate of increase in the operating-force relating amount islarge is decreased more than when the rate of increase in the operatingforce relating amount is small. Thus, the degree of opening of thethrottle valve is decreased only when it is necessary.

[0036] In the above-mentioned invention, the throttle-opening-degreecontrol device may further include increasing-rate-related limited-timeopening-degree decreasing means for decreasing the degree of opening ofthe throttle valve only for a limited time when the negative pressure inthe first pressure chamber is smaller than a predetermined necessarynegative pressure determined according to the rate of increase in theoperating-force relating amount.

[0037] Additionally, there is provided according to another aspect ofthe present invention a vacuum booster apparatus adapted to be used fora brake apparatus provided in a vehicle having an internal combustionengine, the vacuum booster apparatus comprising:

[0038] a vacuum booster having a first pressure chamber and a secondpressure chamber, the first pressure chamber being connected to anintake pipe of the engine on a downstream side of a throttle valve, thesecond pressure chamber being selectively connectable to one of thefirst pressure chamber and atmosphere, the vacuum booster boosting anoperating force applied to a brake operating member based on a pressuredifference between the first pressure chamber and the second pressurechamber; and

[0039] a throttle-opening-degree control device controlling a negativepressure in the first pressure chamber by controlling a degree ofopening of the throttle valve,

[0040] wherein the throttle-opening-degree control device includesoperating-time opening-degree-decrease suppressing means for suppressinga decrease in the degree of opening of the throttle valve so that thedecrease in the degree of opening of the throttle valve when the brakeoperating member is being operated is smaller than when the brakeoperating member is not being operated.

[0041] When a braking operation is being performed, the second pressurechamber is alternately connected to either the first pressure chamber oratmosphere. Accordingly, the negative pressure in the first pressurechamber is decreased as the braking operation continues. If the negativepressure in the first pressure chamber is sufficiently increased beforethe braking operation is performed, that is, when a braking operation isnot being performed, the negative pressure in the first pressure chambercan be maintained at a sufficient level for a long time. That is, thenegative pressure in the first pressure chamber can be prevented frombeing smaller than the predetermined necessary negative pressure if thenegative pressure in the first pressure chamber is increased when abraking operation is not being performed. Accordingly, a frequency ofoperations for decreasing the degree of opening of the throttle valvecan be deceased when a braking operation is being performed.

[0042] As discussed above, if the degree of opening of the throttlevalve is decreased when a braking operation is being performed, that is,the ultra lean burn is performed, a rate of fuel consumption isincreased. However, according to the vacuum booster apparatus of thisinvention, since a frequency of operations for decreasing the degree ofopening of the throttle valve is decreased, an increase in the rate offuel consumption can be suppressed.

[0043] Additionally, when the negative pressure in the first pressurechamber is increased when a braking operation is being performed, abraking force is increased if the operating force applied to the brakeoperating member by a driver is constant. In order to maintain aconstant braking force, the driver must control the operating force.This deteriorates a brake feel given to the driver. In order toeliminate such an inconvenience, in the vacuum booster apparatusaccording to this invention, a frequency of operations for decreasingthe degree of opening of the throttle valve is decreased so that theoperation for decreasing the degree of opening of the throttle valve israrely performed when a braking operation is being performed. Thus,there is an advantage in that deterioration of a braking feel isprevented.

[0044] In the vacuum booster apparatus according to the above-mentionedinvention, the throttle-opening-degree control device may includeopening-degree decreasing means for decreasing the degree of opening ofthe throttle valve when the negative pressure in the first pressurechamber is smaller than a preset negative pressure, and theoperating-time opening-degree-decrease suppressing means may includeoperating-time preset-negative-pressure decreasing means for decreasingthe preset negative pressure so that the preset negative pressure whenthe brake operating member is being operated is smaller than that whenthe brake operating member is not being operated.

[0045] When a braking operation is not being performed, the degree ofopening of the throttle valve is decreased when the negative pressure inthe first pressure chamber is smaller than a non-operating-time presetnegative pressure. When a braking operation is being performed, thedegree of opening of the throttle valve is decreased when the negativepressure in the first pressure chamber is smaller than an operating-timepreset negative pressure, which is smaller than the non-operating-timepreset negative pressure. Since the operating-time preset negativepressure is smaller than the non-operating-time preset negativepressure, a start condition to start a decreasing operation for thedegree of opening of the throttle valve when a braking operation isbeing performed tends to be unsatisfied as compared to that when abraking operation is not being performed. That is, a frequency ofoperations for decreasing the degree of opening of the throttle valve isdecreased since the non-operating-time preset negative pressure is setto be a relatively large value.

[0046] Each of the operating-time preset negative pressure and thenon-operating-time preset negative pressure may be either a fixed valueor a variable value determined according to an operational state of thebrake operating member.

[0047] For example, the operating-time preset negative pressure may beset to a value at which the vacuum booster does not reach its boostlimit when the brake operating member is operated by a normal operatingforce, and the non-operating-time preset negative pressure may be set toa value at which the vacuum booster does not reach its boost limit evenwhen the negative pressure in the first pressure chamber becomes smalldue to the brake operating member being operated by a normal operatingforce. In such a case, the non-operating-time preset negative pressureis set to be greater than the operating-time preset negative pressure byan amount of decrease in the negative pressure when a normal brakingoperation is performed.

[0048] Additionally, the operating-time preset negative pressure may beset to the above-mentioned predetermined necessary negative pressure.The non-operating-time preset negative pressure may be a fixed value ora variable value. For example, the non-operating-time preset negativepressure may be set to a value greater than a maximum operating-timepreset negative value during a previous braking operation by theabove-mentioned amount of decrease in the negative pressure when anormal braking operation is performed. Additionally, thenon-operating-time preset negative pressure may be set to a valuecorresponding to a running state of the vehicle. That is, thenon-operating-time preset negative pressure when a vehicle speed islarge may be set to be greater than that when the vehicle speed is low,or the non-operating-time preset negative pressure when a vehicle isrunning up a slope may be set to be greater than that when the vehicleis running down a slope.

[0049] Additionally, in the vacuum booster apparatus according to theabove-mentioned invention, the operating-time preset-negative-pressuredecreasing means may include operating-time preset-negative-pressuredetermining means for determining the preset negative pressure used whenthe brake operating member is being operated in accordance with at leastone of an operating-force relating amount related to the operating forceapplied to the brake operating member and a rate of increase in theoperating-force relating amount.

[0050] When a braking operation is being performed, the degree ofopening of the throttle valve is decreased only when the negativepressure in the first pressure chamber is smaller than theoperating-time preset negative pressure (hereinafter referred to as avariable preset negative pressure) which is determined based on at leastone of the operating-force relating amount and the rate of increase inthe operating-force relating amount. When the variable preset negativepressure is smaller than the operating-time preset necessary negativepressure (hereinafter referred to as a fixed preset negative pressure)which is previously determined as mentioned above, a frequency ofoperations for decreasing the degree of opening of the throttle valvecan be further decreased.

[0051] Additionally, the operating-time preset negative pressure may beselectively set to one of the variable preset negative pressure and thefixed preset negative pressure. For example, if the degree of opening ofthe throttle valve is decreased when the negative pressure in the firstpressure chamber becomes smaller than the one of the variable presetnegative pressure and the fixed preset negative pressure which isgreater than the other, the vacuum booster can be prevented fromreaching its boost limit when the brake operating member is beingoperated with a large operating force. In such a case, the fixed presetnegative pressure can be determined to be a minimum negative pressure inthe first pressure chamber.

[0052] Additionally, there is provided according to another aspect ofthe present invention a brake apparatus adapted to be provided in avehicle having an internal combustion engine, the brake apparatuscomprising:

[0053] a vacuum booster having a first pressure chamber and a secondpressure chamber, the first pressure chamber being connected to anintake pipe of the engine on a downstream side of a throttle valve, thesecond pressure chamber being selectively connectable to one of thefirst pressure chamber and atmosphere, the vacuum booster boosting anoperating force applied to a brake operating member based on a pressuredifference between the first pressure chamber and the second pressurechamber;

[0054] a braking-force assisting apparatus increasing a braking force toa value greater than a value corresponding to an output of the vacuumbooster; and

[0055] a throttle-opening-degree control device controlling a degree ofopening of the throttle valve, the throttle-opening-degree controldevice including abnormal-time throttle-opening-degree decreasing meansfor decreasing the degree of opening of the throttle valve, when anabnormality occurs in the braking force assisting apparatus, so as toincrease a negative pressure in the first pressure chamber.

[0056] In the above-mentioned invention, the braking-force assistingapparatus may increase a braking force after the vacuum booster reachesits boost limit or before the vacuum booster reaches its boost limit. Asfor the braking-force assisting device which increases a braking forcebefore the vacuum booster reaches its boost limit, there is a type whichincreases a braking force when an emergency braking operation isperformed and a type which increases a braking force a predeterminedperiod before the vacuum booster reaches its boost limit. If the vacuumbooster is a type in which a boost ratio changes before the vacuumbooster reaches its boost limit, a braking force is increased at a timewhen the boost ratio is changed. In any case, in a brake apparatusprovided with a braking-force assisting apparatus, a necessity fordecreasing the degree of opening of the throttle valve is high when anabnormality occurs in the braking-force assisting apparatus, and is lowwhen the braking-force assisting apparatus is normal.

[0057] The braking-force assisting apparatus may comprise a reservoir, apump which pressurizes the brake fluid in the reservoir and provides thepressurized brake fluid to a brake cylinder, a motor for driving thepump, a pressure control device which can control the fluid pressure inthe brake cylinder by the brake fluid discharged by the pump andbraking-force controlling means for controlling the fluid pressure inthe brake cylinder by controlling the motor and the pressure controldevice. When an abnormality occurs in the motor or the pressure controldevice, or when the an abnormality occurs in an electric system such asin the braking-force controlling means, it is determined that anabnormality occurs in the braking-force assisting apparatus. Anabnormality occurring in the braking-force assisting apparatus can bedetected by an abnormality detecting device.

[0058] A description will now be given of a case in which a brakingforce is increased after the vacuum booster reaches substantially anatmospheric pressure.

[0059] When the braking-force assisting apparatus is normal, a brakingforce can be increased after the vacuum booster reaches its boost limit.However, when an abnormality occurs in the braking force assistingapparatus, a braking force cannot be increased. In such a case, theboost limit of the vacuum booster can be increased by increasing thenegative pressure in the first pressure chamber. If the brake operatingmember is operated with a large operating force, the vacuum booster isprevented from reaching its boost limit and the operating force isboosted by the vacuum booster which results in an increase in thebraking force. On the other hand, when the braking-force assistingapparatus is normal, the braking force can be increased after the vacuumbooster reaches its limit without increasing the boost limit of thevacuum booster. Thereby, the negative pressure in the first pressurechamber is not required to be increased.

[0060] When the braking-force assisting apparatus is abnormal, thedegree of opening of the throttle valve may be decreased when thenegative pressure in the first pressure chamber is smaller than thepreset negative pressure, or may be decreased either when the negativepressure in the first pressure chamber is smaller or greater than thepreset negative pressure. The above-mentioned preset negative pressuremay be a predetermined fixed value or a variable value which isdetermined by the operating-force relating amount related to theoperating force applied to the brake operating member.

[0061] The braking-force assisting apparatus may be any apparatus whichincreases a braking force after the vacuum booster reaches its boostlimit. That is, the braking-force assisting apparatus may be anapparatus which increases a braking force to a value corresponding to anoperating force applied to the brake operating member, or may be anapparatus which increases a braking force independent of an operatingforce applied to the brake operating force. Additionally, the fact thatthe vacuum booster has reached its boost limit can be detected by aboost-limit detecting device which detects the pressure in the secondpressure chamber having reached an atmospheric pressure or detects themaster cylinder pressure having reached a boost limit which can beobtained according to the negative pressure in the first pressurechamber.

[0062] It should be noted that even in the brake apparatus according tothe present invention, a decrease of the degree of opening of thethrottle valve cannot be completely omitted when the braking-forceassisting apparatus is normal. When the braking force assistingapparatus is abnormal, the degree of opening of the throttle valve isdecreased due to an abnormality occurring in the braking-force assistingapparatus. The degree of opening of the throttle valve may be decreased,when the braking-force assisting apparatus is normal, due to othercauses such as a case in which the negative pressure in the firstpressure chamber is smaller than the predetermined necessary negativepressure which is determined according to the operating-force relatingamount related to the operating force applied to the brake operatingforce. In such a case, when the braking-force assisting apparatus isabnormal, the degree of opening of the throttle valve is decreased bythe abnormal-time throttle opening degree decreasing means, and when thebraking-force-assisting apparatus is normal, the degree of opening ofthe throttle valve is decreased by the limited-time opening-degreedecreasing means. In a case in which a braking force is increased afterthe vacuum booster reaches its boost limit, if the boost limit isincreased when the braking force assisting apparatus is normal, afrequency of the vacuum booster reaching its boost limit can bedecreased which results in a decrease in a frequency of operation of thebraking-force assisting apparatus.

[0063] In the brake apparatus according to the present invention, thebraking-force assisting apparatus may increase the braking force after apressure in the second pressure chamber reaches substantially anatmospheric pressure, and the abnormal-time throttle-opening-degreedecreasing means may determine the degree of opening of the throttlevalve in accordance with a target boost limit.

[0064] According to this invention, the degree of opening of thethrottle valve which degree is decreased when the braking-forceassisting apparatus is abnormal, is determined by the abnormal-timethrottle-opening-degree determining means. During an abnormal time inwhich the throttle valve is maintained at a degree of opening, thenegative pressure in the first pressure chamber when the abnormal-timethrottle opening degree is small can be greater than that when theabnormal-time throttle opening degree is large. That is, theabnormal-time throttle-opening degree when the target boost limit(corresponds to the negative pressure in the first pressure chamber) islarge is determined to be smaller than that when the target boost limitis small.

[0065] Additionally, together with or instead of the abnormal-timethrottle opening degree, a decreased-state maintaining time which is aperiod for maintaining a state in which the degree of opening of thethrottle valve is small may be determined. A level of the negativepressure in the first pressure chamber is not increased, immediatelyafter the degree of opening of the throttle valve is decreased, to alevel corresponding to the degree of opening of the throttle valve.Accordingly, the negative pressure in the first pressure chamber and theboost limit when the decreased-state maintaining time is long is greaterthan that when the decreased-state maintaining time is short. In such acase, the abnormal-time throttle-opening-degree decreasing meansincludes opening-degree decreased-state-maintaining determining means.

[0066] In the brake apparatus according to the present invention, thebraking-force assisting apparatus may increase the braking force after apressure in the second pressure chamber reaches substantially anatmospheric pressure, and the abnormal-time throttle-opening-degreedecreasing means decreases the degree of opening of the throttle valvewhen the negative pressure in the first pressure chamber is smaller thana predetermined necessary negative pressure which is determined inaccordance with at least one of an operating-force relating amountrelated to the operating force applied to the brake operating member anda rate of increase in the operating-force relating amount.

[0067] In the brake apparatus according to this invention, the degree ofopening of the throttle valve is decreased when the braking-forceassisting apparatus is abnormal and when the negative pressure of thefirst pressure chamber is smaller than the predetermined necessarynegative pressure. That is, if the braking-force assisting apparatus isabnormal, the negative pressure in the first pressure chamber is notrequired to be increased when the vacuum booster does not reach itsboost limit or when a possibility for the vacuum booster reaching itsboost limit is low. Thus, in the brake apparatus according to thepresent invention, a frequency of operations for decreasing the degreeof opening of the throttle valve can be decreased.

[0068] Additionally, in the brake apparatus according to the presentinvention, the abnormal-time throttle-opening-degree decreasing meansmay decrease the degree of opening of the throttle valve when the brakeoperating member is not being operated and when the negative pressure inthe first pressure chamber is smaller than a non-operating-time presetnegative pressure, and the abnormal-time throttle opening degreedecreasing means may also decrease the degree of opening of the throttlevalve when the brake operating member is being operated and when thenegative pressure in the first pressure chamber is smaller than anoperating-time preset negative pressure which is smaller than thenon-operating-time preset negative pressure.

[0069] As mentioned above, the degree of opening of the throttle valvedoes not have to be decreased immediately after an abnormality occurs inthe braking-force assisting apparatus, and may be decreased when thenegative pressure in the first pressure chamber becomes smaller than thenon-operating time preset negative pressure or the operating-time presetnegative pressure.

[0070] Additionally, in the brake apparatus according to the presentinvention, the throttle-opening-degree control device may includenormal-time conditioned throttle-opening-degree decreasing means fordecreasing the degree of opening of the throttle valve when thebraking-force assisting apparatus is normal and when the negativepressure in the first pressure chamber satisfies a predeterminedcondition.

[0071] According to this invention, the degree of opening of thethrottle valve is decreased by the abnormal-time throttle-opening-degreedecreasing means when the braking force assisting apparatus is abnormal,and the degree of opening of the throttle valve is decreased by thenormal-time conditioned throttle-opening-degree decreasing means whenthe braking-force assisting apparatus is normal. The normal-timeconditioned throttle-opening-degree decreasing means may include atleast one of the limited-time opening-degree decreasing means, thelarge-change-time opening-degree decreasing means and the operating-timeopening-degree-decrease suppressing means.

[0072] In a case in which the braking-force assisting apparatusincreases a braking force after the pressure in the second pressurechamber reaches substantially an atmospheric pressure, a frequency ofoperations of the braking-force assisting apparatus can be decreased ifthe boost limit of the vacuum booster is increased by decreasing thedegree of opening of the throttle valve when the braking-force assistingapparatus is normal. Thus, when the braking force assisting apparatusincludes a pump, an operating noise of the pump can be reduced.

[0073] Additionally, there is provided according to another aspect ofthe present invention a brake apparatus adapted to be provided in avehicle having an internal combustion engine, the brake apparatuscomprising:

[0074] a vacuum booster having a first pressure chamber and a secondpressure chamber, the first pressure chamber being connected to anintake pipe of the engine on a downstream side of a throttle valve, thesecond pressure chamber being selectively connectable to one of thefirst pressure chamber and atmosphere, the vacuum booster boosting anoperating force applied to a brake operating member based on a pressuredifference between the first pressure chamber and the second pressurechamber so as to output the boosted operating force to a mastercylinder;

[0075] a brake cylinder operated by a fluid pressure generated by themaster cylinder;

[0076] a pressure-increasing device increasing a fluid pressure providedto the brake cylinder to be greater than the fluid pressure generated bythe master cylinder; and

[0077] a throttle-opening-degree control device controlling a degree ofopening of the throttle valve, the throttle-opening-degree controldevice including abnormal-time throttle-opening-degree decreasing meansfor decreasing the degree of opening of the throttle valve, when anabnormality occurs in the pressure-increasing device, so as to increasea negative pressure in the first pressure chamber.

[0078] The above-mentioned pressure-increasing device can be abraking-force assisting apparatus which increases a braking force afterthe vacuum booster reaches its boost limit or an emergency-timebraking-force assisting apparatus which increases a braking force at anemergency time. It is preferable that in a case in which theemergency-time braking-force is used, when the emergency-time brakingforce assisting apparatus is abnormal, the negative pressure in thefirst pressure chamber be set to be greater than a non-operating-timepreset negative pressure used when a braking operation is not beingperformed. This is because a braking force can be increased when a largeoperating force is applied by a driver at an emergency time.

[0079] Other objects, features and advantages of the present inventionwill become more apparent from the following detailed description whenread in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0080]FIG. 1 is a structural diagram of a brake apparatus including avacuum booster apparatus according to a first embodiment of the presentinvention;

[0081]FIGS. 2A and 2B are illustrations for explaining a structure andan operation of a pressure control valve shown in FIG. 1;

[0082]FIG. 3 is a structural diagram of an engine provided with athrottle valve which controls a negative pressure used by a vacuumbooster shown in FIG. 1;

[0083]FIG. 4 is a graph showing a relationship between a negativepressure and a boost limit in the vacuum booster shown in FIG. 1;

[0084]FIG. 5 is a graph showing a relationship between a pressure in abrake cylinder and a force applied to a brake pedal;

[0085]FIG. 6 is a graph showing a relationship between a pressuredifference and a master cylinder pressure;

[0086]FIG. 7 is a graph for explaining an output of the vacuum boosterwhen a negative pressure is increased;

[0087]FIG. 8 is a flowchart of a throttle-opening-degree controloperation;

[0088]FIG. 9 is a flowchart of a process in step S14 of FIG. 8;

[0089]FIG. 10 is a flowchart of a braking-force control operation;

[0090]FIG. 11 is a diagram for explaining an operation of the vacuumbooster;

[0091]FIG. 12 is a flowchart of an operation according to athrottle-opening-degree control program stored in a ROM of a brakecontrol apparatus included in the brake apparatus shown in FIG. 1;

[0092]FIG. 13 is a flowchart of an operation according to athrottle-opening-degree control program stored in a ROM of a brakecontrol apparatus included in a brake apparatus according to a secondembodiment of the present invention; and

[0093]FIG. 14 is a graph showing a relationship between a throttleopening degree and a target pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0094] A description will now be given, with reference to FIG. 1, of afirst embodiment of the present invention. FIG. 1 is a structuraldiagram of a brake apparatus including a vacuum booster apparatusaccording to the first embodiment of the present invention.

[0095] The brake apparatus shown in FIG. 1 comprises a vacuum boosterapparatus 6 and a braking-force assisting apparatus 8. A vacuum booster12 included in the vacuum booster apparatus 8 has a limit of boost.Considering such a limit of boost, the braking-force assisting apparatus8 controls a braking force so that a deceleration of a vehicle increaseswith an ideal slope relative to a brake operating force. Thebraking-force assisting apparatus 8 is operated by a pump 40. In thebrake apparatus, an untilock control can be performed by using the pump40 so as to prevent a tendency of lock of each wheel from beingexcessive when a braking operation is being performed.

[0096] In FIG. 1, a brake pedal 10 corresponds to a brake operatingmember. The brake pedal 10 is connected to a master cylinder 14 via thevacuum booster 12.

[0097] In the vacuum booster 12 (hereinafter referred to as booster 12),a power piston is movably provided within a housing having an innerspace so as to form an airtight seal with an inner wall of the housing.The inner space of the housing of the vacuum booster 12 is divided bythe power piston into a negative pressure chamber 12 a corresponding toa first pressure chamber and a pressure changing chamber 12 bcorresponding to a second pressure chamber. The negative pressurechamber 12 a is located on the master cylinder 14 side. The pressurechanging chamber 12 b is located on the brake pedal 10 side. Thenegative pressure chamber 12 a is connected to an intake pipe 18 of anengine at a position downstream of a throttle valve 20 as shown in FIG.3.

[0098] The master cylinder 14 is a tandem type which includes twopistons serially arranged and slidably fit within a housing so that thetwo pistons are moved by an output of the booster 12. The mastercylinder 14 provides a the same fluid pressure to each of chambersformed in front of the two pistons. A brake cylinder 26 operating afront left wheel FL and a brake cylinder 26 operating a rear right wheelRR are connected to one of the chambers, and a brake cylinder operatinga front right wheel FR and a brake cylinder operating a rear left wheelRL are connected to the other of the chambers. The brake apparatus canbe of a disc type or a drum type which restricts rotation of a wheel bypressing a friction member against a friction surface of a rotatingmember rotating with the wheel, a pressing force of the friction memberbeing generated by a fluid pressure.

[0099] A fluid passage 24 extends from one of the chambers of the mastercylinder 14. The fluid passage 24 is branched and the brake cylinders 26of the front left and rear right wheels FL and RR are connected torespective ends of the branched fluid passage 24.

[0100] A pressure control valve 30 is provided on a master cylinder 14side of a branched point of the fluid passage 24. The pressure controlvalve 30 controls a fluid pressure on the brake cylinder 26 side of thefluid passage 24 relative to a fluid pressure on the master cylinder 14side. Specifically, if a pressure difference between the brake cylinderpressure and the master cylinder pressure is smaller than a targetpressure difference in a state in which a brake fluid is discharged fromthe pump 40, the pressure control valve 30 prevents a flow of the brakefluid from the pump 40 to the master cylinder 14. On the other hand, ifthe pressure difference tends to be greater than the target pressuredifference, the pressure control valve 30 permits the flow of brakefluid from the pump 40 to the master cylinder 14. Accordingly, the brakecylinder pressure is controlled by the pressure control valve 30 so thatthe brake cylinder pressure is greater than the master cylinder pressureand the pressure difference therebetween is equal to the target pressuredifference.

[0101] The pressure control valve 30 of the present embodiment is a typethat electromagnetically controls the pressure difference betweenpressures in the brake cylinders 26 and the master cylinder 14.Specifically, as shown in FIGS. 2A and 2B, the pressure control valve 30comprises a housing (not shown in the figure), a valve body 70, a valveseat 72 on which the valve body 70 sits and a solenoid 74 whichgenerates a magnetic force so as to move the valve body 70 relative tothe valve seat 72. The valve body 70 controls a state of the brake fluidflow in the fluid passage 24 between the master cylinder 14 and thebrake cylinder 26.

[0102] In a non-operated state (OFF state) in which the solenoid 74 isnot activated, the valve body 70 is separated from the valve seat 72 bya spring force of a spring 76. Thereby, flow of the brake fluid in thefluid passage 24 between the master cylinder 14 and the brake cylinders26 is permitted in both directions. As a result, when a brakingoperation is performed, the brake cylinder pressure is changed inresponse to a change in the master cylinder pressure. During the brakingoperation, a force is exerted on the valve body 70 in a direction inwhich the valve body 70 is separated from the valve seat 72.Accordingly, as long as the solenoid 74 is not activated, the valve body70 does not sit on the valve seat 72 even if the master cylinderpressure, that is, the brake cylinder pressure, is increased. That is,the pressure control valve 30 is a normally open valve.

[0103] On the other hand, in an operated state (ON state) in which thesolenoid 74 is activated, an armature 78 is attracted by a magneticforce of the solenoid 74, and the valve body 70 (movable member) whichmoves together with the armature 78 is seated on the valve seat 72(stationary member). At this time, as shown in FIG. 2B, the attractingforce F1 and a force of a sum of forces F2 and F3 are exerted on thevalve body 70 in opposite directions. The attracting force Fl isgenerated by the magnetic force generated by the solenoid 74. The forceF2 is generated based on a pressure difference between the brakecylinder pressure and the master cylinder pressure. The spring force F3is generated by the spring 76. A magnitude of the force can berepresented by a product of the pressure difference between the brakecylinder pressure and the master cylinder pressure and an effectivepressure receiving area of the valve body 70 which area receives thebrake cylinder pressure.

[0104] In the operated state (ON state) in which the solenoid 74 isactivated, and the discharge pressure of the pump 40, that is, the brakecylinder pressure, is sufficiently low such that a relationship F2≦F1−F3is established, the valve body 70 is maintained to be seated on thevalve seat 72. That is, the valve body 70 is maintained to be seated onthe valve seat 72 when the pressure difference force F2 does not exceedthe sum of the attracting force Fl and the spring force F3. In such astate, the brake fluid is prevented from flowing from the pump 40 to themaster cylinder 14. Accordingly, the discharge pressure of the pump 40is increased, and a fluid pressure higher than the master cylinderpressure is generated in the brake cylinders 26.

[0105] If the discharge pressure of the pump 40, that is, the brakecylinder pressure, is further increased such that a relationshipF2>F1−F3 is established, the valve body 70 is separated from the valveseat 72. That is, when the sum of the pressure difference force F2 andthe spring force F3 exceeds the attracting force F1, the valve body 70is separated from the valve seat 72, and the brake fluid is providedfrom the pump 40 to the master cylinder 14. As a result, the dischargepressure of the pump 40, that is, the brake cylinder pressure, isprevented from being further increased. Accordingly, if the spring forceF3 of the spring 76 is not considered, a brake cylinder pressure greaterthan the master cylinder pressure by the attracting force F1 of thesolenoid is generated.

[0106] Additionally, the pressure control valve 30 is designed so that amagnitude of the attracting force F1 of the solenoid 74 changes linearlyin response to a magnitude of an activating current I of the solenoid74.

[0107] As shown in FIG. 1, a bypass passage 82 is provided to thepressure control valve 30. A check valve 84 is provided in the middle ofthe bypass passage 82. The check valve 84 is provided so as to keep afluid flow from the master cylinder 14 to the brake cylinders 26 even ifthe pressure control valve 30 is closed due to a force exerted on themovable member in the pressure control valve 30 when the brake pedal 10is being pressed. Additionally, a relief valve 86 is provided parallelto the check valve 84 so as to prevent a pressure built by the dischargepressure of the pump 40 from being excessive.

[0108] Pressure increasing valves 90, each of which is a normally opensolenoid valve, are provided to the branched portion of the fluidpassage 24 so as to achieve a pressure increasing state in which thebrake fluid is prevented from flowing from the master cylinder 14 to thebrake cylinders 26 when the pressure increasing valves 90 are closed. Abypass passage 92 is connected to each of the pressure increasing valves90, and a check valve 94 is provided to each of the bypass passages 92.A reservoir passage 96 extends from a position between each of thepressure increasing valves 90 and the respective brake cylinder 26, andthe reservoir passage reaches a reservoir 98. A pressure decreasingvalve 100, which is a normally closed solenoid valve, is provided in themiddle of each reservoir passage 96 so as to achieve a pressuredecreasing state in which a flow of the brake fluid from the brakecylinders 26 to the reservoir 98 is permitted when the pressuredecreasing valve 100 is open.

[0109] The reservoir 98 comprises a housing and a reservoir piston 104slidably fit in the housing so a to form a substantially airtight sealwith an inner wall of the housing. A reservoir chamber 106 is formed bythe housing and the reservoir piston 104 so as to store the brake fluidin the reservoir chamber 106 under a pressure generated by a spring 108.

[0110] The reservoir 98 is connected to an inlet of the pump 40 via aninlet passage 110. An output of the pump 40 is connected to a portion ofthe fluid passage 24 between the pressure control valve 30 and thepressure increasing valves 90 via an outlet passage 114. An inlet valve116 which is a check valve is provided to the inlet passage 110. Anoutlet valve 118 which is a check valve is provided to the outletpassage 114. Additionally, an orifice 120 and a fixed damper 122 areprovided to the outlet passage 114 so as to reduce pulsation in thebrake fluid pressure.

[0111] In the present embodiment, a supplementary passage 130 isprovided to connect the master cylinder 14 and to the reservoir 98. Aflow control valve 140 is provided to the supplementary passage 130. Theflow control valve 140 is a normally closed solenoid valve which isopened when the brake fluid should be provided from the master cylinder14 to the reservoir 98 and when the pump 40 is being operated so thatthe brake fluid flows from the master cylinder 14 to the reservoir 98.When it is not required to provide the brake fluid from the mastercylinder 14 to the reservoir 98, the flow control valve 140 is closed soas to prevent the brake fluid from flowing from the master cylinder 14to the reservoir 98, which flow permits an increase in the fluidpressure by the master cylinder 14. It is determined whether or not thebrake fluid is required to be introduced from the master cylinder 14based on a determination as to whether or not the brake fluid is presentin the reservoir 98, which brake fluid is to be pumped up by the pump40. When an antilock control is being performed, an accumulation ofperiods during which the pressure increasing valve 90 achieves thepressure increasing state is calculated. Additionally, an accumulationof periods during which the pressure decreasing valve 100 achieves thepressure decreasing state is calulated. An amount of brake fluidremaining in the reservoir 98 is estimated based on the pressureincreasing time and the pressure decreasing time. As mentioned above,the flow control valve 140 is opened when the pump 40 is being operatedand there is no brake fluid to be pumped up by the pump 40. Thereby, thebrake fluid is prevented from being provided from the master cylinder 14to the reservoir before the master cylinder pressure is sufficientlyincreased. Additionally, the reservoir 98 is prevented from storing anexcessive amount of brake fluid due to the brake fluid in the mastercylinder 14 being directly suctioned during an antilock control.

[0112] The supplementary passage 130 does not directly connect themaster cylinder 14 and the reservoir 98. That is, the supplementarypassage 130 is connected to a middle portion of the inlet passage 110. Acheck valve 142 is provided between the middle portion and the reservoir98 so as to prevent the brake fluid from flowing from the supplementarypassage 130 to the reservoir 98. During a braking operation, before thebrake fluid in the master cylinder 14 is provided to the reservoir 98through the check valve 142 by the flow control valve 30 being opened,the brake fluid in the reservoir 98 can be pumped up by the pump 40.Accordingly, a quick response of the pump 40 can be achieved thanpumping the brake fluid after a pressure in the reservoir 98 isdecreased. Additionally, a load applied to the pump 40 is reduced whichfacilitates a reduction in a capacity of the pump 40.

[0113] The brake apparatus is controlled by a brake control apparatus144. The brake control apparatus 144 comprises a microcomputer. An inputof the micro computer is connected to a brake switch 150. Other inputsof the microcomputer are connected to a master-cylinder-pressure sensor152, wheel speed sensors 156 and an abnormality detecting device 158.The brake switch 150 detects whether the brake pedal 10 is in anoperated state. The master-cylinder-pressure sensor 152 detects themaster cylinder pressure. The wheel speed sensor 156 detects arotational speed of each wheel. The abnormality detecting device 158detects whether the braking-force assisting apparatus 8 is normal orabnormal. An output of the microcomputer is connected to the solenoid 74of the pressure control valve 30 and a solenoid of each of the solenoidvalves 90, 100 and 140 via respective drive circuits. A ROM provided inthe brake control apparatus 144 stores various sets of informationincluding table information shown in FIG. 4 which table informationindicates a relationship between a negative pressure in the negativepressure chamber 12 a and a limit of boost, programs for controlling adegree of opening of the throttle valve 20 represented by flowchartsshown in FIGS. 8 and 9 and programs for controlling a braking forcerepresented by a flowchart shown in FIG. 10.

[0114] The above-mentioned throttle valve 20 is provided in the engineshown in FIG. 3. The engine shown in FIG. 3 is a direct injectiongasoline engine which is provided with a high-pressure swirl injector164 so as to directly inject gasoline into a cylinder 166. A piston 168is moved within the cylinder 166 due to combustion of gasoline, and,thereby, a crank 170 is driven.

[0115] The above-mentioned intake pipe 18 is connected to the cylinder166 so that air is introduced into the cylinder 166 through the intakepipe 18. The throttle valve 20 and a swirl valve 174 are provided in theintake pipe 18. A vacuum hose 22 is connected to the intake pipe betweenthe throttle valve 20 and the swirl valve 174. The throttle valve 20 isoperated by a throttle motor 176 so that a degree of opening of thethrottle valve 20 is controlled by controlling an operation of thethrottle motor 176. An amount of intake air introduced into the cylinder166 is controlled by controlling a degree of opening of the throttlevalve 20. A negative pressure in the negative pressure chamber 12 a ofthe booster 12 is also controlled by controlling the degree of openingof the throttle valve 20. The degree of opening of the throttle valve 20is detected by a throttle position sensor 178. Similarly, the swirlvalve 174 is controlled by an operation of a swirl motor 180. The degreeof opening of the swirl valve 174 is detected by an SCV position sensor182. The degree of opening of the swirl valve 174 is controlled so thata swirl of air (air flow) in the cylinder 166 is optimum for a state ofcombustion.

[0116] An exhaust pipe 190 is also connected to the cylinder 166. A gasgenerated in the combustion chamber is exhausted to atmosphere via athree-state catalytic converter.

[0117] An intake valve 192 and an exhaust valve 194 are provided to theintake pipe 18 and an opening of the exhaust pipe 190 at the cylinder166, respectively. The intake valve 192 and the exhaust valve 194 areoperated by rotation of a camshaft. In the present embodiment, a timingof the intake valve 192 is controllable.

[0118] An exhaust gas cleaning device is provided to a connectingpassage which connects between the exhaust pipe 190 and the intake pipe18 by bypassing the cylinder 166 so as to recirculate an exhaust, whichis controlled in response to an operational state of the engine so thatan amount of nitrogen oxide NOx is reduced. An amount of exhaust gas tobe recirculated is controlled by an exhaust gas recirculation (EGR)valve 196.

[0119] The high-pressure swirl injector 164 is connected to a fuel tank208 via a low-pressure pump 202, a high-pressure pump 204 and ahigh-pressure pipe 206. The gasoline stored in the fuel tank 208 ispumped by the low-pressure pump 202 and is pressurized by thehigh-pressure pump 204 so that a predetermined amount of pressurizedgasoline is provided to the high-pressure swirl injector 164. Thehigh-pressure swirl injector 164 is provided with a swirl nose fromwhich gasoline is injected into the cylinder 166 so that the injectedgasoline becomes fine mist. A fuel pressure sensor 210 is mounted in themiddle of the high-pressure pipe 206 so as to detect a pressure of thegasoline to be provided to the high-pressure swirl injector 164.Additionally, gasoline pumped by the low-pressure pump 202 is providedto a low-temperature injector 212 provided to a middle portion of theintake pipe 18.

[0120] The high-pressure pump 204 is driven by a cam which operates theintake valve 192 so that gasoline is provided in association with anoperation of the cam. The high-pressure pump 204 includes a solenoidvalve so that an amount of gasoline delivered by the high-pressure pump204 is controllable according to a state of activation of a solenoid ofthe solenoid valve.

[0121] The engine shown in FIG. 3 is controlled by an engine controlapparatus 220. The engine control apparatus 220 comprises amicrocomputer. Inputs of the microcomputer are connected to the throttleposition sensor 178, the SCV position sensor 182 and the fuel pressuresensor 210. Additionally, an accelerator position sensor 230 whichdetects a degree of opening of an acceleration pedal 228, a crankposition sensor 232 which detects a position of the crank 170 and abooster negative pressure sensor 234 which detects a negative pressurein the negative pressure chamber 12 a of the booster 12 are connected toinputs of the microcomputer. An output of the microcomputer is connectedto the throttle motor 176 and the swirl motor 180 via respective drivecircuits (not shown in the figure).

[0122] An engine revolution speed is detected based on a crank positiondetected by the crank position sensor 232. A drive torque desired by adriver is determined based on a degree of opening of the acceleratordetected by the accelerator position sensor 230. A ROM provided in theengine control apparatus 220 stores various programs including programsfor controlling the degree of opening of the throttle valve 20. In thepresent embodiment, an amount of fuel injected by the high-pressureswirl injector 164, the degree of opening of the throttle valve 20, adegree of opening of the swirl valve 174, the degree of opening of theEGR valve 196, an intake valve timing and an ignition timing arecontrolled based on the engine revolution speed and a degree of openingof the accelerator.

[0123] In the engine associated with the present embodiment, the ultralean burn (stratified charge combustion) is performed in a state inwhich the engine speed is not very high (in a state in which an engineload is not very large). On the other hand, a uniform combustion isperformed in a state in which the engine speed is high (in a state inwhich the engine load is large). In the uniform combustion, a combustionis performed at an air-fuel ratio near a stoichiometric air-fuel ratio.However, in the ultra lean burn, a combustion is performed at a highair-fuel ratio of about 25 to 50. In a state in which an engine load isnot very large, a necessary torque can be output even when the ultralean burn occurs. During a braking operation, since an engine load isnot very large, the ultra lean burn is performed in many cases.According to the ultra lean burn, a rate of fuel consumption is reducedwhich results in energy saving. During the ultra lean burn, a degree ofopening of the throttle valve 20 is set to a relatively large valuebecause a large amount of air is required to increase a air-fuel ratioin order to prevent misfire.

[0124] However, since the negative pressure chamber 12 a of the booster12 is connected to the downstream side of the throttle valve 20, anegative pressure in the negative pressure chamber 12 a is decreasedwhen the degree of opening of the throttle valve 20 is set to arelatively large value. If the brake pedal 10 is pressed in a state inwhich the negative pressure of the negative pressure chamber 12 a issmall, the booster 12 reaches its boost limit even if the operatingforce to the brake pedal 10 is not very large. This cannot provide asufficient boost action. As shown in FIG. 4, when the negative pressureis decreased, the boost limit is also decreased. In the presentembodiment, to increase the negative pressure of the negative pressurechamber 12 a, the degree of opening of the throttle valve 20 isdecreased. When the degree of opening of the throttle valve 20 isdecreased while the ultra lean burn is performed, the combustion isstepwisely or gradually changed from the ultra lean burn to a uniformcombustion so that the degree of opening of the throttle valve 20 isdecreased in a state of uniform combustion. This is because if thedegree of opening of the throttle valve 20 is decreased during the ultralean burn, a state of combustion becomes unstable due to misfire or thelike. Additionally, when the same output torque is output, a amount ofinjected fuel, the degree of opening of the throttle valve 20, thedegree of opening of the swirl valve, and the degree of opening of theEGR valve 196 are much different between the ultra lean burn and theuniform combustion. Accordingly, if these values are rapidly changed,drivability is deteriorated. Additionally, combustion is changed to theuniform combustion when an abnormality occurs in the hydraulic brakeapparatus. This includes a case in which an abnormality occurs in thebraking-force assisting apparatus 8. Additionally, the state of theengine is stepwisely or gradually changed from the state of uniformcombustion to the state of ultra lean combustion.

[0125] The engine control apparatus 220 is connected to the brakecontrol apparatus 144 via a communication device so that exchange ofinformation is performed therebetween. Negative pressure informationwhich represents a negative pressure in the negative pressure chamber 12a detected by the booster negative pressure sensor 234 is provided fromthe engine control apparatus 220 to the brake control apparatus 144.Opening-degree decreasing information which instructs the engine controlapparatus 220 to decrease of a degree of opening of the throttle valve20 and opening-degree control permitting information which instructs theengine control apparatus 220 to control the degree of opening areprovided from the brake control apparatus 144 to the engine controlapparatus 220. The opening-degree control permitting information isprovided when there is no need to decrease the degree of opening. Asmentioned above, when the degree of opening of the throttle valve 20 isdecreased during the ultra lean burn, a state of combustion must bechanged to the uniform combustion. Accordingly, there is a problem inthat a rate of fuel consumption is increased. In order to eliminate sucha problem, in the present embodiment, a frequency of decreasing thedegree of opening of the throttle valve 20 during the ultra lean burn isdecreased.

[0126] A description will now be given of an operation of the brakeapparatus shown in FIG. 1.

[0127] When the brake pedal 10 is pressed, an operating force to thebrake pedal 10 is boosted by the booster 12, and is transmitted to themaster cylinder 14. In the master cylinder 14, a fluid pressurecorresponding to the operating force is generated in each of thepressurizing chambers, and the generated pressure is transmitted to thebrake cylinders 26. If a tendency of locking of the left and right frontwheels is excessive, an antilock control is performed so as to maintainan appropriate brake slip state. That is, the pressure in the brakecylinders 26 is controlled by the solenoid valves 90 and 100 accordingto the pressure of the brake fluid discharged by the pump 40. It shouldbe noted that the pressure in the brake cylinders 26 may be controlledbased on a pressure of the brake fluid in the master cylinder 14.

[0128] Additionally, when the operating force applied to the brake pedal10 is increased and the booster reaches its boost limit, a pump motor160 is operated. Accordingly, the pressure in the brake cylinders 26 isincreased so as to be controlled to a value corresponding to theoperating force by a control of the solenoid 74 of the pressure controlvalve 30. A pressure higher than the master cylinder pressure PM by apressure difference AP is generated in the brake cylinders 26. Thepressure difference AP is an increased amount of the brake cylinderpressure PB relative to the master cylinder pressure PM. Accordingly, asshown in FIG. 5, the braking force can be increased with a uniformincreasing slope before and after the boost limit of the booster 12 isreached. A relationship between the pressure difference AP and themaster cylinder pressure PM is as shown in a graph of FIG. 6. A currentsupplied to the solenoid 74 of the pressure control valve 30 iscontrolled so that the pressure difference shown in FIG. 6 is achieved.

[0129] The vacuum booster 12 cannot boost the operating force applied tothe brake-operating member after a pressure in the pressure changingchamber 12 b increases from a negative pressure and reaches anatmospheric pressure. If the braking force is increased by a deviceother than the booster 12 after the boost limit is reached, the brakingforce can be increased with an appropriate slope (for example, with asubstantially uniform slope). Thus, the braking force can be increasedwith a stable slope irrespective of whether or not the boost limit isreached. Thus, safeness and a feel of a braking operation can beimproved.

[0130] In the present embodiment, the braking-force assisting apparatus8 is constituted by the reservoir 98, the pump 40, the pump motor 160,the pressure control valve 30, the flow control valve 140, the pumpmotor 160 of the brake control apparatus 144 and units for controllingthe pressure control valve 30 and the flow control valve 140.

[0131] Additionally, a determination as to whether or not the booster 12reaches its boost limit can be made based on the master cylinderpressure PM and the negative pressure in the negative pressure chamber12 a. Since the boost limit of the booster 12 is determined by apressure difference between the pressures in the negative pressurechamber 12 a and the pressure changing chamber 12 b when the pressure inthe pressure changing chamber 12 b reaches an atmospheric pressure, theboost limit PS of the booster 12 can be estimated based on the negativepressure in the negative pressure chamber 12 a as shown in FIG. 4. Ifthe master cylinder pressure PM reaches the boost limit PS, it can bedetermined that the boost function of the booster 12 has reached itslimit. In the present embodiment, a boost-limit detecting unit isconstituted by the master-cylinder-pressure sensor 152, the boosternegative pressure sensor 234 and parts for determining whether or notthe booster 12 of the brake control apparatus 144 reaches its boostlimit.

[0132] A control of the braking-force assisting apparatus 8 is performedin accordance with a braking-force control program represented by aflowchart of FIG. 10.

[0133] In step S1, it is determined whether or not the booster 12reaches its boost limit. That is, it is determined whether or not themaster cylinder pressure PM is equal to or greater than the boost limitPS. If it is determined that the boost limit has been reached, theroutine proceeds to step S2 so as to perform a pressure-increasingcontrol. On the other hand, if it is determined that the boost limit hasnot been reached, the routine proceeds to step S3 so as to perform thenon-pressure-increasing control.

[0134] In the pressure-increasing control, a current is supplied to thesolenoid 74 of the pressure control valve 30 so that the pressuredifference ΔP is achieved and the flow control valve 140 is opened ifnecessary. Additionally, the pump motor 160 is driven. On the otherhand, in the non-pressure-increasing control, the current supplied tothe solenoid 74 of the pressure control valve 30 is set to zero, and theflow control valve 140 is closed. Additionally, the pump motor 160 isstopped. It should be noted that the pump motor 160 may be drivenaccording to the antilock control program.

[0135] As mentioned above, when the braking force assisting apparatus 8is normal, the braking force after the booster 12 reaches the boostlimit can be increased with the same slope as the slope before thebooster limit is reached. However, when an abnormality occurs in thebraking-force assisting apparatus 8, the braking force cannot beincreased with the same slope. In order to eliminate such a problem,when an abnormality is detected in the braking-force assisting apparatus8 and when the negative pressure in the negative pressure chamber 12 ais less than a predetermined negative pressure described later, theopening-degree decreasing information is output to the engine controlunit 220 so as to decrease the degree of opening of the throttle valve20. When the negative pressure in the negative pressure chamber 12 a isincreased, the boost limit of the booster 12 can be increased. Thus, ifthe brake pedal 10 is operated by a large operating force, the brakingforce can be sufficiently boosted by the booster 12. An abnormality ofthe braking-force assisting apparatus 8 can be detected by theabnormality detecting device 158. In the present embodiment, theabnormality detecting device 158 detects an abnormality occurring in anelectric system. It should be noted that the abnormality detectingdevice 158 may detect an abnormality occurring in a hydraulic systemsuch as fluid leakage in the inlet passage 110 or the outlet passage114.

[0136] The above-mentioned predetermined negative pressure is set to bea non-operating-time preset negative pressure β1 during a non-operatingtime, and is set to one of an operating-time preset negative pressure β2and a necessary negative pressure, whichever is greater, during anoperating time. The operating-time preset negative pressure β2 issmaller than the non-operating-time preset negative pressure β1, thatis, the operating-time preset negative pressure β2 is closer to anatmospheric pressure than the non-operating-time preset negativepressure β1. Accordingly, a frequency of operations for decreasing thedegree of opening of the throttle valve 20 when the brake pedal 10 isbeing operated is smaller than that when the brake pedal 10 is not beingoperated. When the brake pedal 10 is not being operated, the negativepressure chamber 12 a and the pressure changing chamber 12 b areconnected to each other so that the pressure in the pressure changingchamber 12 b is equal to the pressure in the negative pressure chamber12 a. When the brake pedal 10 is operated, the pressure changing chamber12 b is alternately connected to either the negative pressure chamber 12a or atmosphere. Thereby, the negative pressure in the negative pressurechamber 12 a is decreased. However, if the negative pressure issufficiently increased when the brake pedal is not operated, a conditioncan be established that the booster 12 does not reach its boost limiteven when the negative pressure is decreased while the brake pedal 10 isoperated. In the present embodiment, the non-operating-time presetnegative pressure β1 is set to a value (closer to an absolute vacuum),which is greater than a value at which the boost limit is not reachedwhen a brake operation is performed with a normal operating force bymore than an amount of decrease in the negative pressure when a normalbraking operation is performed. Additionally, the operating-time presetnegative pressure β2 is set to a value at which the boost limit is notreached by a normal braking operation.

[0137] During a braking operation, since an engine load is not verylarge, as mentioned above, an ultra lean combustion is performednormally. However, when the degree of opening of the throttle valve 20is decreased, the state of combustion must be changed to a uniformcombustion. This causes a problem in that a rate of fuel consumption isincreased. On the other hand, when a braking operation is not performed,there is a case in which the uniform combustion is performed. Thus, arate of fuel consumption may not be increased if the degree of openingof the throttle valve 20 is decreased. Accordingly, an increase in arate of fuel consumption can be suppressed by decreasing the degree ofopening of the throttle valve 20 when the brake pedal 10 is not beingoperated rather than when the brake pedal 10 is being operated.

[0138] When the negative pressure is increased when the brakingoperation is performed, a braking force is increased if an operatingforce applied to the brake pedal 10 by a driver is maintained constantand a stroke of the brake pedal is increased as shown in FIG. 7. On thecontrary, in order to maintain the braking force at a constant value,the driver must decrease the operating force applied to the brake pedal10 which deteriorates a brake feel. However, in the present embodiment,a frequency of operations for decreasing the degree of opening of thethrottle valve 20 is decreased when the braking operation is performed,and, thereby, deterioration in the brake feel can be suppressed.

[0139] The above-mentioned necessary negative pressure is determinedbased on the master cylinder pressure PM as an operating force relatingamount which relates to an operating force applied to the brake pedal 10and a rate of increase in the master cylinder pressure PM as a rate ofincrease of the operating force relating amount.

[0140] The necessary negative pressure is set to a greater value whenthe master cylinder pressure PM is large than when the master cylinderpressure PM is small. This is because the boost limit of the booster 12must be increased more when the master cylinder pressure PM is largethan when the master cylinder pressure PM is small since the booster 12is closer to its boost limit when the master cylinder pressure PM islarge than when the master cylinder pressure PM is small. If thenecessary negative pressure is increased and the negative pressure inthe negative pressure chamber 12 a is increased, the boost limit of thebooster 12 is increased, which condition allows boosting an operatingforce by the same ratio.

[0141] The necessary negative pressure is set larger when a rate ofincrease in the master cylinder pressure PM is large than when the rateof increase in the master cylinder pressure PM is small. This is becausethe negative pressure must be increased when the rate of increase islarge so as to maintain a sufficient operating speed of the booster 12.Additionally, a possibility for reaching the boost limit is higher whenthe rate of increase is large than when the rate of increase is small.If the master cylinder pressure PM is equal, the possibility forreaching the boost limit is high when the rate of increase is largesince an operating force is rapidly increased. On the other hand, whenthe rate of increase is small, the possibility for reaching the boostlimit is low. If the rate of increase is a negative value and when theoperating force is being released, the possibility for reaching theboost limit is further decreased.

[0142] In the present embodiment, the negative pressure in the negativepressure chamber 12 a is not directly compared with the necessarynegative pressure but a comparison is made between the boost limit and avalue (hereinafter referred to as an operational state value) obtainedby adding a boost-limit-arrival estimating value (hereinafter referredto as an arrival estimating value) corresponding to the rate of increaseto the master cylinder pressure PM. If the degree of opening of thethrottle valve 20 is controlled so that a state in which the boost limitis larger than the operational state value is maintained, the negativepressure in the negative pressure chamber 12 a is maintained to begreater than the necessary negative pressure. As mentioned above, avalue of the master cylinder pressure PM corresponds to the operatingforce applied to the brake pedal 10; the arrival estimating valuecorresponds to the rate of increase in the master cylinder pressure PM;and a value of the boost limit corresponds to the negative pressure inthe negative pressure chamber 12 a. Accordingly, a comparison betweenthe negative pressure in the negative pressure chamber 12 a and thenecessary negative pressure corresponds to a comparison between theoperational state value and the boost limit. The arrival estimatingvalue is set to a value α1 when the rate of increase in the mastercylinder pressure PM is a positive value, and is set to a value a2 whichis smaller than the value al when the rate of increase is a negativevalue. Since the possibility for rapidly reaching its boost limit ishigher when the rate of increase is large than when the rate of increaseis small, the arrival estimating value is increased when the rate ofincrease is large. The operational state value is large when theoperating-force relating amount is large and the rate of increase in theoperating force is large.

[0143] When the boost limit is greater than the operational state value,the booster 12 has not reached its boost limit, and, thus, it can bedetermined that the possibility for reaching the boost limit is low. Insuch a case, the negative pressure is greater than the necessarynegative pressure, and the degree of opening of the throttle valve 20 isnot required to be decreased. When the boost limit is smaller than thenecessary negative pressure, it can be determined that the boost limithas been reached or the possibility for rapidly reaching the boost limitis high. In such a case, the negative pressure is smaller than thenecessary negative pressure, and the degree of opening of the throttlevalve 20 is required to be decreased. Thus, the opening-degreedecreasing information is output to the engine control apparatus 220.

[0144] As mentioned above, if the negative pressure in the negativepressure chamber 12 a is maintained to be greater than the necessarynegative pressure, the boost limit of the booster 12 can be increased soas to increase a braking force even if an abnormality occurs in thebraking-force assisting apparatus 8.

[0145] In the present embodiment, the degree of opening of the throttlevalve 20 is decreased during the non-operating time, so that during theoperating time the negative pressure in the negative pressure chamber 12a is not decreased below the predetermined negative pressure which isdetermined to be the greater one of the necessary negative pressuredetermined as mentioned above and the operating-time preset negativepressure β2. That is, when the necessary negative pressure is smallerthan the operating-time preset negative pressure β2, the negativepressure in the negative pressure chamber 12 a is set so as not to besmaller than the operating-time preset negative pressure β2. On theother hand, when the necessary negative pressure is greater than theoperating-time preset negative pressure β2, the negative pressure in thenegative pressure chamber 12 a is set so as not to be smaller than thenecessary negative pressure.

[0146] A description will now be given, with reference to FIG. 8, of athrottle-opening-degree control operation.

[0147] First, in S11, it is determined whether or not the brake pedal 10is being operated based on an output signal of the brake switch 150. Ifthe brake pedal 10 is being operated, the routine proceeds to step S12.In step S12, it is determined whether or not there is an abnormality inthe braking-force assisting apparatus 8 by the abnormality detectingdevice 158. If it is determined that there is an abnormality in thebraking-force assisting apparatus 8, the routine proceeds to step S13.In step S13, it is determined whether or not a negative pressure in thenegative pressure chamber 12 a is smaller than the operating-time presetnegative pressure β2. The negative pressure in the negative pressurechamber 12 a is known by the negative pressure information provided fromthe engine control apparatus 220. If the negative pressure is largerthan the operating-time preset negative pressure β2, that is, if thedetermination is negative, the routine proceeds to step S14. In stepS14, it is determined whether or not the negative pressure in thenegative pressure chamber 12 a is greater than the necessary negativepressure corresponding to the operating force applied to the brake pedal10. If the negative pressure is smaller than the necessary negativepressure, the routine proceeds to step S15 so as to output theopening-degree decreasing information. On the other hand, if thenegative pressure is greater than the necessary negative pressure, theroutine proceeds to step S16 so as to output the opening degree controlpermitting information.

[0148] If it is determined, in step S13, that the negative pressure inthe negative pressure chamber 12 a is smaller than the operating-timepreset negative pressure β2, that is, the determination in step S13 isaffirmative, the routine proceeds to step S15 so as to output theopening-degree decreasing information. Additionally, if it isdetermined, in step S12, that the braking-force assisting apparatus 8 isnormal, the routine proceeds to step S16. That is, if the braking-forceassisting apparatus 8 is normal, the opening-degree decreasinginformation is not output. In this case, a braking force is increased bythe braking-force assisting apparatus 8 after the booster 12 reaches itsboost limit as mentioned above.

[0149] Additionally, if it is determined, in step S11, that the brakepedal is not being operated, the routine proceeds to step S17. In stepS17, it is determined whether or not the negative pressure is smallerthan the non-operating-time preset negative pressure β1. If the negativepressure is smaller than the non-operating-time preset negative pressureβ1, the routine proceeds to step S15 so as to output the opening-degreedecreasing information. Otherwise, the routine proceeds to step S16.

[0150] A description will now be given, with reference to FIG. 9, of thedetermination in step S14 as to whether or not the negative pressure inthe negative pressure chamber 12 a is greater than the necessarynegative pressure.

[0151] First, in step S41, the boost limit PS is obtained based on thenegative pressure in the negative pressure chamber 12 a, the informationof the negative pressure being provided by the engine control apparatus220. The boost limit PS can be obtained according to a relationshipbetween the boost limit PS and the negative pressure which is stored astable information shown in FIG. 4. In step S42, it is determined whetheror not a rate of increase in the master cylinder pressure PM is equal toor greater than zero. If the rate of increase is equal to or greaterthan zero, the routine proceeds to step S43. In step S43, the arrivalestimating value α is set to the value al. On the other hand, if therate of increase is less than zero, the routine proceeds to step S44 soas to set the arrival estimating value a to the value α2. Then, in stepS45, the boost limit PS is compared with the operation state value whichis obtained by adding the arrival estimating value a to the mastercylinder pressure PM.

[0152] If the boost limit PS is smaller than the operation state value,the routine proceeds to step S15 of FIG. 8 so as to output theopening-degree decreasing information. On the other hand, if the boostlimit PS is equal to or greater than the operational state value, theroutine proceeds to step S16 of FIG. 8 so as to output theopening-degree control permitting information.

[0153] A description will now be given, with reference to FIG. 11, of aspecific example of the above-mentioned control operation. When thebrake pedal 10 is not being operated and when the negative pressure inthe negative pressure chamber 12 a is greater than thenon-operating-time preset negative pressure β1, the opening-degreedecreasing information is not output. However, when the negativepressure is decreased to a value smaller than the non-operating-timepreset negative pressure β1, the opening-degree decreasing informationis output. When the brake pedal 10 is not being operated, the negativepressure is maintained to a value greater than the non-operating timepreset negative value β1.

[0154] If the brake pedal 10 is operated at a time t1, the negativepressure is decreased. However, if the negative pressure is maintainedto be greater than the non-operating-time preset negative pressure β1during the non-operating time, the booster 12 rarely reaches its boostlimit when a normal braking operation is performed. Accordingly, theoperating force applied to the brake pedal 10 can be boosted with aconstant ratio. When the brake pedal 10 is being operated, the negativepressure is controlled so that the negative pressure is not smaller thanthe operating-time preset negative pressure β2. However, if thenecessary negative pressure is greater than the operating-time presetnegative pressure β2, the negative pressure is controlled so as not tobe smaller than the necessary negative pressure.

[0155] When the brake pedal 10 is operated along a single dashed chainline M, the operational state value is changed along a single dashedchain line M′. In a range where the operating force is not very large,the operational state value M′ cannot be greater than the operating-timepreset negative pressure β2. Accordingly, the degree of opening of thethrottle valve 20 is controlled so that the negative pressure in thenegative pressure chamber 12 a is not decreased to a value smaller thanthe operating-time preset negative pressure β2.

[0156] When the brake pedal 10 is operated with a large operating force,that is, when the brake pedal 10 is operated along a double dashed chainline N, the necessary negative pressure exceeds the operating-timepreset negative pressure β2 at a time t2. Accordingly, the negativepressure is increased so that the boost limit is not decreased to avalue smaller than the operational state value N′.

[0157] As mentioned above, if the negative pressure in the negativepressure chamber 12 a is maintained to be greater than the necessarynegative pressure, the boost limit can be increased in response to theoperating force relating amount. Accordingly, if the brake pedal 10 isoperated with a large operating force, the operating force can beappropriately boosted by the booster 12. Since the negative pressure inthe negative pressure chamber 12 a cannot be an absolute vacuum, theboost limit cannot be a infinitely large value. However, if anabnormality occurs in the braking-force assisting apparatus 8, the boostlimit can be increased so that the master cylinder pressure can beincreased to increase the braking force. In the present embodiment, whenthe braking operation is being performed, the opening-degree decreasinginformation is output only when it is necessary. That is, theopening-degree decreasing information is output only when an abnormalityoccurs in the braking-force assisting apparatus 8 and when the negativepressure in the negative pressure chamber 12 a is greater than one ofthe operating-time preset negative pressure β2 and the necessarynegative pressure, which one is greater than the other. Accordingly, thedegree of opening of the throttle valve 20 is positively decreased whenit is necessary, and is not decreased unnecessarily. Thus, a frequencyof decreasing operations for decreasing the degree of opening of thethrottle valve 20 can be decreased so that deterioration in a rate offuel consumption due to a decrease in the degree of opening of thethrottle valve 20 when the ultra lean burn is performed can besuppressed. Additionally, since a change in the negative pressurecorresponding to the operating force when the braking operation is beingperformed, deterioration in a brake feel can be suppressed.

[0158] It should be noted that, in the above-mentioned embodiment,although the negative pressure is set not to be smaller than one of theoperating-time preset negative pressure β2 and the necessary negativepressure, which one is greater than the other, the operating-time presetnegative pressure β2 is not necessarily considered and the negativepressure may be merely maintained so as not to be smaller than thenecessary negative pressure as shown in the flowchart of FIG. 12. Insuch a case, the boost limit corresponding to the negative pressure iscontrolled not to be smaller than the operating force relating amount M′or N′. In this case, it can be considered that the operating-time presetnegative pressure β2 corresponds to the necessary negative pressure (avariable value). However, in this case, the non-operating-time presetnegative pressure β1 must be set to a value which is not exceeded by theoperating-time preset negative pressure β2. That is, thenon-operating-time preset negative pressure β1 must be set to a valuegreater than a value obtained by adding an amount of decrease in thenegative pressure when a braking operation is performed to a value atwhich the boost limit is not reached when the brake pedal 10 is beingoperated with a large operating force. Alternatively, the necessarynegative pressure may not be considered and the negative pressure may becontrolled so as not to be smaller than the operating-time presetnegative pressure β2. Additionally, the negative pressure may becontrolled so as not to be smaller than one of the necessary negativepressure and the operating-time preset negative pressure β2, which oneis smaller than the other. In any case, a frequency of decreasingoperations for decreasing the degree of opening of the throttle valve 20can be decreased, and deterioration in a brake feel can be suppressed.

[0159] Additionally, the necessary negative pressure may be determinedbased on only the master cylinder pressure PM or only a rate of increasein the master cylinder pressure PM, or may be determined by consideringother conditions. That is, the operational state value may be set to avalue of the master cylinder pressure PM itself. Additionally, thearrival estimating value a may be continuously changed based on a rateof increase in the master cylinder pressure PM.

[0160] Further, values of the non-operating-time preset negativepressure β1 and the operating-time preset negative pressure β2 are notlimited to the values described above, and other values may be used. Forexample, the operating-time preset negative pressure β2 may be set to avalue at which the boost limit is not reached when the brake pedal isoperated with a large operating force, and the non-operating-time presetnegative pressure β1 is set to be a value greater than a value obtainedby adding an amount of decrease in the negative pressure when the brakepedal 10 is operated with a large operating force one time to theoperating-time preset negative pressure β2. Additionally, theoperating-time preset negative pressure β2 may be set to a minimum valuewhich is needed for the negative pressure chamber 12 a.

[0161] Additionally, the negative pressure may not necessarily bemaintained to be greater than the non-operating-time preset negativepressure β1 when a braking operation is not performed. In practice, thedegree of opening of the throttle valve 20 may be decreased when thenegative pressure is needed, that is, only when the negative pressure inthe negative pressure chamber 12 a becomes smaller than the necessarynegative pressure. Additionally, the degree of opening of the throttlevalve 20 may be decreased only when a braking operation is not beingperformed. As mentioned above, if the negative pressure in the negativepressure chamber 12 a is sufficiently increased when a braking operationis not being performed, there may be a case in which it is not necessaryto decrease the degree of opening of the throttle valve 20 when abraking operation is being performed.

[0162] Additionally, in the above-mentioned embodiment, when anabnormality occurs in the braking force assisting apparatus 8, it isdetermined whether or not the opening-degree decreasing condition issatisfied so that the opening-degree decreasing information is outputwhen the opening-degree decrease condition is satisfied. However, whenan abnormality occurs in the braking-force assisting apparatus 8, thedegree of opening of the throttle valve 20 may be decreased by a valuecorresponding to a target boost limit (corresponding to a targetnegative pressure of the negative pressure chamber 12 a). The targetboost limit can be determined based on at least one of the mastercylinder pressure and a rate of increase in the master cylinderpressure. For example, according to a graph shown in FIG. 14, a degreeof opening may be increased when the target boost limit is small, andthe degree of opening may be decreased when the target boost limit islarge. In the present embodiment, the degree of opening of the throttlevalve 20 is controlled according to execution of athrottle-opening-degree control program represented by the flowchart ofFIG. 13.

[0163] In FIG. 13, it is determined, in step S61, whether or not abraking operation is being performed. If a braking operation is beingperformed, the routine proceeds to step S62. In step S62, it isdetermined whether or not an abnormality occurs in the braking forceassisting apparatus 8. If it is determined that an abnormality occurs inthe braking-force assisting apparatus 8, the routine proceeds to stepS63. In step S63, the degree of opening of the throttle valve 20 isdetermined according to the target negative pressure, and informationfor instructing a decrease of the degree of opening of the throttlevalve 20 to the determined degree of opening is output. In such a case,opening-degree information which represents the degree of opening isoutput together with or instead of the opening-degree decreasinginformation. Additionally, an opening-degree decreasing time whichrepresents a period for decreasing the degree of opening may be obtainedbased on the target boost limit so that opening-degree decreasing timeinformation is output.

[0164] Additionally, the negative pressure may be controlledirrespective of whether the braking-force assisting apparatus 8 isnormal or abnormal so that the negative pressure does not become smallerthan the necessary negative pressure. If the negative pressure iscontrolled so as not to be smaller than the necessary negative pressure,an appropriate boost can be achieved if a braking force is large whichresults in a braking force corresponding to an operating force.

[0165] If the boost limit is increased when the braking force assistingapparatus 8 is normal, an operation of the brake force assistingapparatus 8 can be delayed. This provides an advantage in that a drivingnoise of the pump 40 is reduced.

[0166] Additionally, the braking-force assisting apparatus 8 may beoperated when an emergency braking operation is required instead ofbeing operated after the booster limit is reached. In such a case, it isdetermined whether or not a large braking force is required based on anoperational speed of the brake pedal 10 so that the braking-forceassisting apparatus 8 is operated so as to increase a braking force whenit is determined that the a large braking force is required. In such acase, it is preferable that the negative pressure be sufficientlyincreased even when a braking operation is not being performed.Additionally, the braking force assisting apparatus 8 is not limited tothe above-mentioned structure, and other structures may be used. Forexample, the brake-force assisting apparatus 8 may include anaccumulator. Additionally, the pressure control valve 30 of the brakingforce assisting apparatus may be replaced with a simple solenoid valve.A pressure in the brake cylinders 26 may be controlled by operations ofthe pressure-increasing valves 90 and the pressure-decreasing valves100, or may be controlled by an operation of the pump motor 160.Further, the braking-force assisting apparatus 8 is not essential. Thatis, the boost limit can be increased by increasing the negative pressurewithout using the braking-force assisting apparatus 8.

[0167] The present invention is not limited to the specificallydisclosed embodiments, and variations and modifications may be madewithout departing from the scope of the present invention.

[0168] The present application is based on Japanese priority applicationNo.10-105119 filed on Apr. 15, 1998, the entire contents of which arehereby incorporated by reference.

What is claimed is:
 1. A vacuum booster apparatus adapted to be used fora brake apparatus provided in a vehicle having an internal combustionengine, said vacuum booster apparatus comprising: a vacuum boosterhaving a first pressure chamber and a second pressure chamber, saidfirst pressure chamber being connected to an intake pipe of said engineon a downstream side of a throttle valve, said second pressure chamberbeing selectively connectable to one of said first pressure chamber andatmosphere, said vacuum booster boosting an operating force applied to abrake operating member based on a pressure difference between said firstpressure chamber and said second pressure chamber; and athrottle-opening-degree control device controlling a negative pressurein said first pressure chamber by controlling a degree of opening ofsaid throttle valve, wherein said throttle-opening-degree control deviceincludes limited-time opening-degree decreasing means for decreasing thedegree of opening of said throttle valve only for a limited time whenthe negative pressure in said first pressure chamber is smaller than apredetermined necessary negative pressure determined by anoperating-force relating amount related to the operating force appliedto said brake operating member.
 2. The vacuum booster apparatus asclaimed in claim 1, wherein said limited-time opening-degree decreasingmeans includes necessary-negative-pressure determining means fordetermining said predetermined necessary negative pressure in accordancewith the operating force relating amount related to the operating forceapplied to said brake operating member.
 3. The vacuum booster apparatusas claimed in claim 1, wherein said limited-time opening-degreedecreasing means includes means for determining said predeterminednecessary negative pressure in accordance with a rate of increase in anoperating-force relating amount related to the operating force appliedto said brake operating member.
 4. The vacuum booster apparatus asclaimed in claim 1, wherein said limited-time opening-degree decreasingmeans decreases the degree of opening of said throttle valve only when aboost limit of said vacuum booster is smaller than an operational statevalue which is determined according to an operating-force relatingamount related to the operating force applied to said brake operatingmember.
 5. A vacuum booster apparatus adapted to be used for a brakeapparatus provided in a vehicle having an internal combustion engine,said vacuum booster apparatus comprising: a vacuum booster having afirst pressure chamber and a second pressure chamber, said firstpressure chamber being connected to an intake pipe of said engine on adownstream side of a throttle valve, said second pressure chamber beingselectively connectable to one of said first pressure chamber andatmosphere, said vacuum booster boosting an operating force applied to abrake operating member based on a pressure difference between said firstpressure chamber and said second pressure chamber; and athrottle-opening-degree control device controlling a degree of openingof said throttle valve in accordance with a negative pressure in saidfirst pressure chamber and at least one of an operating-force relatingamount related to an operating force applied to said brake operatingmember and a rate of increase in the operating-force relating amount. 6.A vacuum booster apparatus adapted to be used for a brake apparatusprovided in a vehicle having an internal combustion engine, said vacuumbooster apparatus comprising: a vacuum booster having a first pressurechamber and a second pressure chamber, said first pressure chamber beingconnected to an intake pipe of said engine on a downstream side of athrottle valve, said second pressure chamber being selectivelyconnectable to one of said first pressure chamber and atmosphere, saidvacuum booster boosting an operating force applied to a brake operatingmember based on a pressure difference between said first pressurechamber and said second pressure chamber; and a throttle-opening-degreecontrol device controlling a negative pressure in said first pressurechamber by controlling a degree of opening of said throttle valve,wherein said throttle opening degree control device includeslarge-change-time opening degree decreasing means for decreasing thedegree of opening of said throttle valve as a rate of increase in anoperating-force relating amount related to the operating force appliedto said brake operating member is increased.
 7. The vacuum boosterapparatus as claimed in claim 6, wherein said throttle-opening-degreecontrol device further includes increasing-rate-related limited-timeopening-degree decreasing means for decreasing the degree of opening ofsaid throttle valve only for a limited time when the negative pressurein said first pressure chamber is smaller than a predetermined necessarynegative pressure determined according to the rate of increase in theoperating-force relating amount.
 8. A vacuum booster apparatus adaptedto be used for a brake apparatus provided in a vehicle having aninternal combustion engine, said vacuum booster apparatus comprising: avacuum booster having a first pressure chamber and a second pressurechamber, said first pressure chamber being connected to an intake pipeof said engine on a downstream side of a throttle valve, said secondpressure chamber being selectively connectable to one of said firstpressure chamber and atmosphere, said vacuum booster boosting anoperating force applied to a brake operating member based on a pressuredifference between said first pressure chamber and said second pressurechamber; and a throttle-opening-degree control device controlling anegative pressure in said first pressure chamber by controlling a degreeof opening of said throttle valve, wherein said throttle-opening-degreecontrol device includes operating-time opening-degree-decreasesuppressing means for suppressing a decrease in the degree of opening ofsaid throttle valve so that the decrease in the degree of opening ofsaid throttle valve when said brake operating member is being operatedis smaller than when said brake operating member is not being operated.9. The vacuum booster apparatus as claimed in claim 8, wherein saidthrottle-opening-degree control device includes opening-degreedecreasing means for decreasing the degree of opening of said throttlevalve when the negative pressure in said first pressure chamber issmaller than a preset negative pressure, and said operating-timeopening-degree-decrease suppressing means includes operating-timepreset-negative-pressure decreasing means for decreasing said presetnegative pressure so that said preset negative pressure when said brakeoperating member is being operated is smaller than when said brakeoperating member is not being operated.
 10. The vacuum booster apparatusas claimed in claim 9, wherein said operating-timepreset-negative-pressure decreasing means includes operating-timepreset-negative-pressure determining means for determining said presetnegative pressure used when said brake operating member is beingoperated in accordance with at least one of an operating-force relatingamount related to the operating force applied to said brake operatingmember and a rate of increase in the operating force relating amount.11. A brake apparatus adapted to be provided in a vehicle having aninternal combustion engine, said brake apparatus comprising: a vacuumbooster having a first pressure chamber and a second pressure chamber,said first pressure chamber being connected to an intake pipe of saidengine on a downstream side of a throttle valve, said second pressurechamber being selectively connectable to one of said first pressurechamber and atmosphere, said vacuum booster boosting an operating forceapplied to a brake operating member based on a pressure differencebetween said first pressure chamber and said second pressure chamber; abraking-force assisting apparatus increasing a braking force to a valuegreater than a value corresponding to an output of said vacuum booster;and a throttle-opening-degree control device controlling a degree ofopening of said throttle valve, said throttle-opening-degree controldevice including abnormal-time throttle-opening-degree decreasing meansfor decreasing the degree of opening of said throttle valve, when anabnormality occurs in said braking-force assisting apparatus, so as toincrease a negative pressure in said first pressure chamber.
 12. Thebrake apparatus as claimed in claim 11, wherein said braking-forceassisting apparatus increases the braking force after a pressure in saidsecond pressure chamber reaches substantially an atmospheric pressure,and said abnormal-time throttle-opening-degree decreasing meansdetermines the degree of opening of said throttle valve in accordancewith a target boost limit.
 13. The brake apparatus as claimed in claim11, wherein said braking force assisting apparatus increases the brakingforce after a pressure in said second pressure chamber reachessubstantially an atmospheric pressure, and said abnormal-time throttleopening-degree-decreasing means decreases the degree of opening of saidthrottle valve when the negative pressure in said first pressure chamberis smaller than a predetermined necessary negative pressure which isdetermined in accordance with at least one of an operating-forcerelating amount related to the operating force applied to said brakeoperating member and a rate of increase in the operating force relatingamount.
 14. The brake apparatus as claimed in claim 11, wherein saidabnormal-time throttle-opening-degree decreasing means decreases thedegree of opening of said throttle valve when said brake operatingmember is not being operated and when the negative pressure in saidfirst pressure chamber is smaller than a non-operating-time presetnegative pressure, and said abnormal-time throttle-opening-degreedecreasing means also decreases the degree of opening of said throttlevalve when said brake operating member is being operated and when thenegative pressure in said first pressure chamber is smaller than anoperating-time preset negative pressure which is smaller than saidnon-operating-time preset negative pressure.
 15. The brake apparatus asclaimed in claim 11, wherein said throttle-opening-degree control deviceincludes normal-time conditioned throttle-opening-degree decreasingmeans for decreasing the degree of opening of said throttle valve whensaid braking-force assisting apparatus is normal and when the negativepressure in said first pressure chamber satisfies a predeterminedcondition.
 16. A brake apparatus adapted to be provided in a vehiclehaving an internal combustion engine, said brake apparatus comprising: avacuum booster having a first pressure chamber and a second pressurechamber, said first pressure chamber being connected to an intake pipeof said engine on a downstream side of a throttle valve, said secondpressure chamber being selectively connectable to one of said firstpressure chamber and atmosphere, said vacuum booster boosting anoperating force applied to a brake operating member based on a pressuredifference between said first pressure chamber and said second pressurechamber so as to output the boosted operating force to a mastercylinder; a brake cylinder operated by a fluid pressure generated bysaid master cylinder; a pressure-increasing device increasing a fluidpressure provided to said brake cylinder to be greater than the fluidpressure generated by said master cylinder; and athrottle-opening-degree control device controlling a degree of openingof said throttle valve, said throttle-opening-degree control deviceincluding abnormal-time throttle-opening-degree decreasing means fordecreasing the degree of opening of said throttle valve, when anabnormality occurs in said pressure-increasing device, so as to increasea negative pressure in said first pressure chamber.